Gas springs are widely used to counterbalance the engine compartment hoods, trunk lids, rear windows, and tailgates of passenger cars, station wagons, and vans to facilitate opening them and to hold them open at or near a fully open position. It is well known that the force outputs of gas springs vary with the temperature of the gas (Boyle's law); at low temperatures, a gas spring produces a force that is significantly less than the force it produces at high temperatures. It is necessary, therefore, to design the gas spring so that it produces a sufficient force to hold the hood, trunk lid or the like (hereinafter referred to as the "load") open at a suitably selected low temperature, for example, -30.degree. C. (Conventionally, the gas spring is designed to produce a force of about one to five pounds over the load at the hold-open position of the load at, say, -30.degree. C.) At high temperatures, the increase in the force output at the hold-open position may be as much as, say, 50 pounds, which means that the force required to move the load from the hold-open position toward closed ("handle load") is 50 pounds. A handle load of that magnitude is obviously undesirable.
U.S. Pat. No. 5,106,065 (Staton, Apr. 21, 1992), which is owned by the assignee of the present invention, describes and shows a gas spring that incorporates a bypass having a spring-biased valve that prevents fluid flow past the piston from the closed end to the rod seal end of the chamber until a predetermined force due to a pressure differential is applied across the piston and a bypass having a thermostatic valve, which opens at a predetermined temperature and permits fluid in the chamber to flow past the piston from the closed end to the rod seal end of the chamber. When the thermostatic valve is closed, the spring-biased valve provides a hold-open force that is combined with the force due to the gas pressure to hold the gas spring against a load (handle load). When the thermostatic valve is open, the hold-open force of the gas spring is that due to the gas pressure alone, inasmuch as the fluid flows across the piston through the bypass with the thermostatic valve.
The gas springs disclosed in U.S. Pat. No. 5,106,065 provide a significant improvement over prior art gas springs in that the variation of the handle load due to temperature changes is considerably reduced. On the other hand, they do not eliminate variations in the handle load, inasmuch as they do not prevent changes in the handle load as functions of temperature but merely establish two ranges of handle loads, a low temperature range in which the thermostatic valve is closed and the spring-biased valve contributes a force increment opposing the handle load and a high temperature range in which that force increment is not applied, the fluid flowing past the piston through the bypass with the then open thermostatic valve. In each range, the force on the rod due to gas pressure varies as a function of the gas pressure changes due to temperature changes. At the high end of each of the two ranges, the force of the gas spring resisting the handle load is considerably higher than it is at the low end.